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.TH GETGROUPLIST 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
getgrouplist \- get list of groups to which a user belongs
.SH SYNOPSIS
.nf
.B #include <grp.h>
.PP
.BI "int getgrouplist(const char *" user ", gid_t " group ,
.BI "                 gid_t *" groups ", int *" ngroups );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR getgrouplist ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    Glibc 2.19 and earlier:
        _BSD_SOURCE
.fi
.SH DESCRIPTION
The
.BR getgrouplist ()
function scans the group database (see
.BR group (5))
to obtain the list of groups that
.I user
belongs to.
Up to
.I *ngroups
of these groups are returned in the array
.IR groups .
.PP
If it was not among the groups defined for
.I user
in the group database, then
.I group
is included in the list of groups returned by
.BR getgrouplist ();
typically this argument is specified as the group ID from
the password record for
.IR user .
.PP
The
.I ngroups
argument is a value-result argument:
on return it always contains the number of groups found for
.IR user ,
including
.IR group ;
this value may be greater than the number of groups stored in
.IR groups .
.SH RETURN VALUE
If the number of groups of which
.I user
is a member is less than or equal to
.IR *ngroups ,
then the value
.I *ngroups
is returned.
.PP
If the user is a member of more than
.I *ngroups
groups, then
.BR getgrouplist ()
returns \-1.
In this case, the value returned in
.IR *ngroups
can be used to resize the buffer passed to a further call
.BR getgrouplist ().
.SH VERSIONS
This function is present since glibc 2.2.4.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR getgrouplist ()
T}	Thread safety	MT-Safe locale
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
This function is nonstandard; it appears on most BSDs.
.SH BUGS
In glibc versions before 2.3.3,
the implementation of this function contains a buffer-overrun bug:
it returns the complete list of groups for
.IR user
in the array
.IR groups ,
even when the number of groups exceeds
.IR *ngroups .
.SH EXAMPLES
The program below displays the group list for the user named in its
first command-line argument.
The second command-line argument specifies the
.I ngroups
value to be supplied to
.BR getgrouplist ().
The following shell session shows examples of the use of this program:
.PP
.in +4n
.EX
.RB "$" " ./a.out cecilia 0"
getgrouplist() returned \-1; ngroups = 3
.RB "$" " ./a.out cecilia 3"
ngroups = 3
16 (dialout)
33 (video)
100 (users)
.EE
.in
.SS Program source
\&
.EX
#include <stdio.h>
#include <stdlib.h>
#include <grp.h>
#include <pwd.h>

int
main(int argc, char *argv[])
{
    int ngroups;
    struct passwd *pw;
    struct group *gr;

    if (argc != 3) {
        fprintf(stderr, "Usage: %s <user> <ngroups>\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    ngroups = atoi(argv[2]);

    gid_t *groups = malloc(sizeof(*groups) * ngroups);
    if (groups == NULL) {
        perror("malloc");
        exit(EXIT_FAILURE);
    }

    /* Fetch passwd structure (contains first group ID for user). */

    pw = getpwnam(argv[1]);
    if (pw == NULL) {
        perror("getpwnam");
        exit(EXIT_SUCCESS);
    }

    /* Retrieve group list. */

    if (getgrouplist(argv[1], pw\->pw_gid, groups, &ngroups) == \-1) {
        fprintf(stderr, "getgrouplist() returned \-1; ngroups = %d\en",
                ngroups);
        exit(EXIT_FAILURE);
    }

    /* Display list of retrieved groups, along with group names. */

    fprintf(stderr, "ngroups = %d\en", ngroups);
    for (int j = 0; j < ngroups; j++) {
        printf("%d", groups[j]);
        gr = getgrgid(groups[j]);
        if (gr != NULL)
            printf(" (%s)", gr\->gr_name);
        printf("\en");
    }

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR getgroups (2),
.BR setgroups (2),
.BR getgrent (3),
.BR group_member (3),
.BR group (5),
.BR passwd (5)
